Content displayed on a web page can be generated by one or more content item servers in response to content item requests that are generated during the rendering of the web page by a client device. Often a publisher of a web page may define multiple content item environments, e.g., advertisement slots, for content items, e.g., advertisements, to be displayed within a web page. Typically, each content item environment has an associated content item request, e.g., an advertisement request, that is transmitted to a content item server, e.g., an advertisement server. When the web page is rendered by a client device that has requested the publisher's web page, a content item will be displayed in each content item environment.
A web page having multiple content item environments can, however, generate excessive traffic to the content item server. Additionally, because web page instructions are often interpreted, the content item requests are generated as each content item request instruction is processed. Accordingly, a first content item request for a web page is transmitted and served without knowledge of any subsequent content item requests that may be encountered for that page. Thus, each content item served by the content item server for a web page is independent of other content items served by the content item server for that page. Accordingly, a suboptimal collection of content items, e.g., advertisements, may be served for a web page if the rendering of the web page generates multiple content item requests.